1. Field of the Invention
The present invention relates to devices and methods for chemical processing. More specifically, the present invention relates to an energy efficient device for the treatment of a gas including the decomposition of chemical compounds within a gas, such as the abatement of pollution within an exhaust gas by the use of an efficient corona discharge plasma reactor.
2. Background
A variety of methods have been investigated for processing chemicals within gases, such as the removal of volatile organic compounds (VOCs) from exhaust gases. One area of study has been the use of electrical discharges within the gas that are designed to interact with the chemicals of concern.
A subset of this field involves the use of corona discharges. A corona or corona discharge is a current discharge between two electrodes with a potential gradient sufficiently high so as to ionize a neutral fluid or gas, creating plasma within the fluid about the electrodes. This plasma state enables the fluid to conduct a charge, even when under other conditions the fluid might be non-conductive.
If one of the electrodes forms a sharp edge or point, then the surrounding fluid will face a higher potential gradient at that area, which can localize plasma formation for a particular applied energy. This feature creates a defined area of conductivity about the edge or point, which can be conductive while other areas in the gas are not. Without such a defined electrode, the potential gradient may not be as high and greater energy may be required for plasma formation. Thus, a corona discharge usually involves two differently shaped electrodes. One electrode may be a needle, a sharp edge, or wire extending in an axial direction. The other electrode may provide a surface proximate to the other electrode, such as a plate or cylinder. Thus, the sharp or defining edge of an electrode can enhance the potential gradient, depending on the application.
Pulsed corona discharges have been used to treat gases, such as the destruction of VOCs. One example of an electrode structure used in VOC abatement is a coaxial geometry, with the center electrode being a wire extending in an axial direction surrounded by a tubular outer electrode. Typically, a dielectric or insulating terminal at each end separates the electrodes and maintains a desired gap or distance. The gas flows along the axis within the tube around the inner electrode. Short high voltage electrical pulses with a fast voltage rise may be applied across the discharge gap between the electrodes. As these pulses are applied across the electrodes a non-homogeneous electric field is created and multiple thin plasma channels or streamers may arise, depending on a number of factors including the type of gas and the pressure. These streamers may arise both in the gas between the electrodes and along the surface of the dielectrics. The pulse duration may be limited to prevent arcing between the electrodes. If a positive charge is placed on the center electrode then the streamer generated will be positive, and will travel from the center electrode or anode to the tubular outer electrode or cathode, forming a positive corona. Other electrode configurations include point-plane, wire-plane, or wire-cylinder. The electrodes are located within a gas discharge chamber also referred to as a plasma reactor. The configurations of such devices may vary, depending on the configuration of the electrodes and the application.
Within the discharge chamber, the plasma ionization produces reactive species, such as radicals or ions and electrons. Positive species will be attracted to a negative electrode while electrons will be attracted in the opposite direction, to a positive electrode. In some cases, a physical configuration or electric field may prevent the recombination of an electron and positive species, preserving it for another purpose. Recombination may be permitted beyond the region of ionization, so that the ionized particles are then attracted to oppositely charged particles or surfaces and recombine. The electric field may accelerate or impart energy to the electrons or radicals within a gas streamer. The high-energy particles can be used to interact with a chemical or pollutant within the gas. For example, a high energy electron may collide with chemical molecule and induce decomposing chemical reactions to produce inert or less toxic chemicals as the gas flows along the discharge chamber.
Effective decomposition of a chemical or pollutant using corona discharges typically requires significant energy consumption. The energy applied across the electrodes is a major contributor to the energy density of the plasma and the population of radical species produced by such devices. In general, the greater the quantity of radical species produced, the greater the likelihood of radical interaction with the contaminant or chemical. Therefore, a technology that increased the efficiency pf corona discharge devices would decrease their operating costs and expand the field of application.